1. Field of the Invention
The present invention relates to a method and apparatus for detecting or transmitting a magnetic field, and more particularly, to a method and apparatus for detecting or transmitting a magnetic field using a detachable magnetic resonator coupled to a simple loop antenna, by which sensitivity is improved, a structure of the apparatus is simplified, and manufacturing costs are reduced.
2. Description of the Related Art
Magnetic resonance imaging (MRI) apparatuses are widely used to diagnose brain diseases, backbone diseases, bones diseases, joint diseases, cardiovascular diseases, chest diseases, congenital cardiac disorder, or myocardial infarction. The MRI apparatus is used to test the tissue type of a human body by detecting a magnetic field of a particular frequency of a hydrogen nucleus and converting a detected magnetic field to a 2D or 3D image. Compared to a computed tomography (CT) or an X-ray test, the MRI apparatus using a magnetic field that is not harmful to the human body is a non-destructive and non-radioactive test method exhibiting a superior contrast and resolution.
When a static magnetic field is applied to the human body, the hydrogen nucleus included in a cell constituent molecule of the human body has a resonant frequency proportional to the strength of the static magnetic field and well absorbs or emits electromagnetic waves of the resonant frequency.
The MRI apparatus is used to observe the interior of the human body according to the above principle. During which a strong static magnetic field of about 1 Tesla is applied to an area to be measured, the MRI apparatus applies an RF pulse wave corresponding to a resonant frequency to the human body so that the hydrogen nucleuses are excited to a high energy status. As the excited hydrogen nucleuses are restored to the original low energy status, most of the energy is externally emitted as a high frequency magnetic field in the same form of the applied RF, which is a magnetic resonance (MR) signal.
When a static magnetic field having a strength varying according to the position of the human body, for example, a gradient magnetic field in which strength increases proportionally to a distance in a particular direction from a reference point, is additionally applied during which the hydrogen nucleuses emit an RF magnetic field, the frequency of the magnetic field emitted from the hydrogen nucleuses at each position has a deviation proportional to the distance, from the center frequency determined by the static magnetic field. By receiving the MR signal using a magnetic field detection apparatus or a receiving antenna and processing the received signal, an image indicating the density distribution of the hydrogen nucleuses are obtained.
Since a more accurate diagnosis is possible as the resolution of an MR image increases, a variety of technologies to increase the resolution have been suggested. In particular, the sensitivity of an MR signal detection apparatus has a great influence on the increase of a resolution.
The MR signal is a narrow band signal and has the center frequency of about 42.6 MHz when a static magnetic field of 1 Tesla is applied. Since the strength of the static magnetic field of a currently widely used equipment is usually between 0.3 Teslas to 3 Teslas, the center frequency is usually between 13 MHz to 130 MHz, and the signal bandwidth is not greater than several hundreds kilohertz. A conventional commercial MR signal detection apparatus is a type of a band pass filtering loop antenna equipped with a resonance circuit having a high quality factor Q and an impedance matching circuit, to have a superior sensitivity characteristic with respect to the MR signal having an RF narrow band magnetic field signal.
FIG. 1A is a photo of a conventional commercial MR signal detection apparatus that is accommodated in a case. FIG. 1B is a photo of an interior circuit of the conventional commercial MR signal detection apparatus. FIG. 1C is a circuit diagram of the conventional commercial MR signal detection apparatus.
Referring to FIG. 1C, the conventional commercial MR signal detection apparatus includes a loop antenna 110 receiving an external magnetic field and a plurality of capacitors 113 and inductors 115 for resonance characteristic and impedance matching. When the strength of an external magnetic field perpendicularly penetrating the surface surrounded by the loop antenna 110 changes, a voltage signal is induced between a signal line 120 and a ground 130. The voltage signal is transmitted to a signal processing unit via a coaxial cable 140 and has a very high sensitivity at a resonant frequency.
FIG. 1D is a graph showing the frequency characteristic of a signal detected by the commercial MR signal detection apparatus at a position 10 cm apart from the a transmitting loop antenna which is driven by a network analyzer and having a flat frequency characteristic. Referring to FIG. 1D, a high transfer characteristic appears at a resonant frequency of about 42.6 MHz, which shows the characteristic of the MR signal detection apparatus when using a static magnetic field of 1 Tesla.
The conventional MR signal detection apparatus equipped with a resonance circuit has problems of a complicated circuit structure and a very high price. Also, since the conventional MR signal detection apparatus has a high sensitivity with respect to only a single narrow band signal, it is difficult to compatibly use the conventional MR signal detection apparatus with other MRI apparatuses having different strengths of static magnetic fields.